There are several plan9 repositories as there are several distribuitions, however I am pretty sure all these keep in sync with the labs (bell labs) code so I would suggest you start with that (unless somone else on this list shouts).
from plan9 its just a case of:
9fs sources
ls /n/sources/plan9/sys/src/cmd/eqn
if not then you should be able to use:
http://plan9.bell-labs.com/sources/plan9/sys/src/cmd/eqn
and a little hacking to trim headers and taililers.
-Steve
